Current limiting device

ABSTRACT

Two electrodes are interconnected in spaced opposed relationship through an electric insulation through which a current limiting material of a self-restoring type extends to normally electrically interconnect both electrodes. The insulation is equal to or higher than the current limiting material in thermal conductivity. Alternatively the current limiting material is partly reduced in cross section and coupled to a pressure relief element. The reduced portion of the current limiting material may be encircled with a material higher in electric resistivity than the limiting material and good in arc-proof property.

United States Patent [191 Ito et al. I

[451 Aug. 14, 1973 CURRENT LIMITING DEVICE [75] Inventors: Toshio Ito; Toshio Mlyamoto; Yuichi Wada; Teijiro Mori, all of City of Amagasaki, Hyogo Prefecture,

Japan [73] Assignee: Mitsubishi Denki Kabushiki Kaisha,

Tokyo, Japan [22] Filed: June 9, 1970 [21] Appl. No.: 44,785

[30] Foreign Application Priority Data 3,599,137 8/1971 Toshio Ito et al. 337/159 X 3,644,860 2/1972 1,316,095 9/1919 Illingworth 337/119 X Primary Examiner-Bernard A. Gilheany Assistant Examiner-F. E. Bell Attorney-Robert E. Burns and Emmanuel J. Lobato [57] ABSTRACT Two electrodes are interconnected in spaced opposed relationship through an electric insulation through which a current limiting material of a self-restoring type extends to normally electrically interconnect both electrodes. The insulation is equal to or higher than the current limiting material in thermal conductivity. Alternatively the current limiting material is partly reduced in cross section and coupled to a pressure relief element. The reduced portion of the current limiting material may be encircled with a material higher in electric resistivity than the limiting material and good in arc-proof property.

3 Claim, 5 Drawing Figures Patented Aug. 14, 1973 3,753,190

CROSS REFERENCES TO RELATED APPLICATIONS US. Pat. No. 3,488,761 entitled Current Limiting Device issued on Jan. 6, 1970 to T. Ito et al. and assigned to the present assignee.

BACKGROUND OF THE INVENTION while repeatedly performing the current limiting opera- I tion with a good reproduciblity.

. SUMMARY oF' THE INVENTION The invention resides in a current limiting device comprising a first metallic electrode and a second nietallic electrode disposed in spaced: opposed relationship an electric insulation disposed so as to mechanically connect thefirst electrode to the'secorid electrode I and electrically insulate the first electrode from the second electrode, an opening extending through at least the electric insulation and disposed between the first and second electrodes, an amount of current limiting material fillingv said opening, said current limiting material being in a selected oneof solid and liquid states at room temperature to maintain the first electrode electrically conductive to the second electrode and" responding to a flow of current-therethroughin excess' of apredetermined magnitude to be evaporated thereby to perform a current limiting operation, at least that portion encircling the current limiting material of the electric insulation being formed of a material at least equalin' thermal conductivity to the current limiting material.

Preferably the opening may be partly reduced in cross sectional area.

Accordingly, it'is an object of the invention to provide a new and improved current limiting device of selfrestoring type having a very great magnitude of overload current endured" by the same. I

It is another object of the invention to provide a new and improved'cu'rrentlimiting device capable of repeatedly performing the current limiting operation with a good reproduciblene'ss while increasing in useful life.

BRIEF DESCRIPTION OF TI-IE DRAWING FIGS. 4 and 5 are schematic longitudinal sectional views of different modifications of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing and particularly to FIG. 1, it is seen that an arrangement disclosed herein comprises a first metallic electrode 10, a second metallic electrode 12 opposing to and partly encircling the first electrode 10 to form a gap therebetween which is filled with an electric insulation 14 serving to maintain both electrodes in place and in electrically insulated relationship, an outer metallic cylindrical member 16 for supporting the second electrode 12 and the insulation 14, and an annular disc 18 of electrically insulating material disposed between the first electrode 10 and the outer cylindrical member 16 to electrically insulate the two from each other. The second electrode 12 is provided on that end portion remote from the first electrode 10 with a recess into which a leading electrode 20 in the form of a container is snugly fitted with an antiarc barrier 22- with a central aperture interposed therebetween. A piston 24 is slidably disposed in the hollow portion of the leading electrode 20 to divide it into two compartments.

As shown in FIG. 1, a longitudinal opening 25 extends centrally through the first electrode 10, the insulation l4 and the second electrode 12 and communicates with that compartmen'tnear to the first electrode 10in the leading electrode 20 through the central aperture on thelb arrier 22. The opening 25 has disposed on that portion near to one end positioned within the first electrode 10 a valve 28 for the 'purpose as will be apparenthereinafter. The other compartment in the leading electrode 20 has a volume of compressed gas 30 tilling the same and acting as a pressure relief element. An O-ring 32 is operatively connected to the piston 24 ensuring that both compartments are hermetically isolated from eachother.

Any suitable current limiting material 26 is-poured into the opening25 through the valve 28 in its open pos'ition until it fills both the opening or passageway 25 andthe compartments communicatingtherewith in the leading electrode 20. Thereafter the valve 28 is put at its closed position. As will be well known, the current limiting'material 26 is in the form of a solid or a liquid at room temperature and responsive to an excessive current such'as ashort-circuitin'g current flowing therethrough to be evaporated in the corresponding vapor high in vapor pressure. The evaporated material presents to that flow of current an electric resistance much higher than the shortcircuiting impedance of the associated circuit thereby to limit the shortcircuiting current below a predetermined magnitude while substa'ntia'lly insulating the first electrode 10 fromthe second electrode 12 for a predetermined period of time. After the completion of the current limiting operation or after both electrodes have been maintained in electrically insulated relationship the evaporated material is rapidly cooled to be solidified or liquidized as the case may be whereupon it is self-restored to its original good electrically conductive state in which both electrodes 10 and 12 are again electrically interconnected. Thus the device is ready for the succeeding operation. Preferred examples of the current limiting material involve sodium (Na), potassium (K), alloys thereof (NaK), gallium (Ga) etc.

Upon evaporating the current limiting material a very high pressure occurs within both the opening 25 and the compartment communicating therewith in the leading electrode 20. This pressure tends to move the piston 24 in the righthand direction as viewed in FIG. 1 to compress the compressed gas 30 in the other compartment. Therefore the pressure is absorbed through the compression of the gas 30 ensuring that the device is prevented from damaging.

With the current limiting material 26 evaporated, the material encircling the evaporated material is subject to a very high temperature environment. Particularly the electric insulation 14 through which the current limiting material 26 extends is required to be formed of a material withstanding at least 1,000 K. Suitable examples of such an insulating material involve ceramics such as alumina (Alberyllia (BeO), LUCALOX (trade mark) etc. Further the insulation 14 may be formed of a heat resisting ceramic consisting essentially of magnesium oxide (MgO), glass, mica etc. by means of a hot pressure molding technique. Such a ceramic may include another metallic salt(s). The heat resisting ceramic is commercially available under the name of Hishilex or Mycalex.

Also one of the performances required for current limiting devices is to permit an overcurrent whose magnitude is equal to from L25 times to 20 times the rated current for the device to flow through the current limiting material for a period of time determined by the particular magnitude of overcurrent without the material evaporated. in order to prevent that portion of the device disposed between the first and second electrodes and 12 respectively from being brought into its electrically isolated state below a predetermined magnitude of current flowing therethrough due to the evaporation of the current limiting material 26, the latter matieral extending through the insulation 14 between the electrodes l0 and 12 is required not to be evaporated by means of a Joules heat generated therein due to an eddy current circulating through the material. This requirement for the current limiting material will now be mathematically discussed with reference to FIG. 2.

FIG. 2 is a model view illustrating the manner in which the current limiting material or element 26 is snugly extending through the insulation 14. As shown in FIG. 2, it is assumed that both the insulation and the current limiting element are in the form of cylinders coaxially disposed and symmetric with respect to their common axis. Also it is assumed that the current limiting element 26 uniformly generates heat through a flow of an overcurrent therethrough. Under the assumed condition the following partial differential equation is held:

where V'2 Laplacian T temperature of the current limiting element 26 K rate of heat emission of the current limiting element 26 t time,

= specific resistance of the current limiting element 26 current flowing through the current limiting element 26 thermal conductivity of current limiting element.

Using a cylindrical coordinate system (r, 4:, z) where the z axis coincides with the common longitudinalaxis of both cylinders 25 and 14, the equation (1) is reduced to M" 0T R J; 0

where Cp specific heat at any given point (r) r specific weight at any given point (r) holding K A/Cpy.

Since the current limiting element and insulation are shown in FIG. 2 as having radii of r, and R respectively, the equation (3) is calculated at where h A rrr A (TI'R2 wr 2)/1rR c m/( PyL In the above equations (5) the suffixes l and 2 denote the current limiting element and the insulation respectively. For example, A, designate the thermal conductivity of the current limiting material and I, designates the specific weight of the insulating material.

Solving the equation (4) yields where l is the axial length of the current limiting element and therefore of the insulation and pi is a thermal time constant expressed by pi 1r (2n 1)P k /Cpy with n being any integer. As shown in FIG. 2, each end face of the cylinder is labelled 1/2 or 1/2 so that the cylindrical coordinate system has it origin at the middle point of the length of the common axis of the cylinders.

From the equation (6) it is seen that arise in temperature is a function of A I, F and Q. Among those parameters only A, and I can be randomly selected. However, the length l is subject to some limitation due to a quantity of heat generated in the current limiting material in view of both the rated current for the particular current limiting device and the thermal severity to which the insulating material is subject upon evaporating the current limiting material. Therefore what can be selected at will is k that will subsequently be mathe-- matically considered.

A first one of the equations (5) will be reduced to From the above equation it is seen that, in order to render A, as large as. possible regardless of the ratio of r,,/R, it is required to render approximately equal to A, (or A, A while A, is large. Also it will readily be seen that with the ratio of r /R. small, a further increase in h requires to hold A, k

Sodium potassium, their alloys etc. used as current limiting material havea thermal conductivity A, in this case A, in theorder of 0. l1 Cal/sec. cm. C. On the other hand, beryllia (BeO) providing an electrically insulating material. has itsthermal conductivity Mof approximately 0 .5 Cal/sec. cm=..C. Therefore a combinationof any one of thecurrent. limiting materials just described with the beryllia approximately fulfils the relationship of A, x,..

While an insulating material such as beryllia inherently high inthermal conductivity is,.of course, effective for use. with the present invention such an insulatingmaterial may be replaced, for example, by any hot. pressure moldingconsisting essentially of mica, glass and a' metallic oxide having'mixed therewith a powdered metalhigh' in thermal conductivity with. satisfactory result. For example, an insulating material available under'the name of Hishilex orMycalex as above described maybe mixed with powdered copper and molded intoany desired magnitude of thermal'conductivity or into'electrically'resistive members for example a thermet as described in the above cited U.S. patent.

FIG. 3 wherein like reference numerals designate the components identicalto those shown in FIG. 1 shows a modification of the device as illustrated in FIG. 1. The arrangement: illustrated is different. from that shown inFIG. 1 only inthati in FIG. 3, theelectric insulation includes one portion 14' formed'of an insulating material only high in thermal conductivity to'directly encircle the current limitingmaterial 26 andthe remaining portions 14 formed of an insulatingmaterial good in heat resisting property and highin thermal conductivity. For example the insulation portion 14- may be formed of a material just described and the'insulationportion l4may be formedof beryllia(B'eO) or'alumina (111,0

FIG. 4 whereinlikeireference numerals designate the componentsidenticalor corresponding to those illustratedinFIG. 1i showsanother modification of the invention. InFlG. 4, the first electrode is rigidly connected inelectrically insulated relationship to the leading electrode. 20 providing the second electrode, through theelectric insulation 14. The current limiting material servingto electrically interconnect both electrodes l0 and 20 partly decreascs'in cross sectional area. Specifically. that end portion 26' of the current limiting material adjacent the first electrode 10 is smaller in cross sectional area than the remaining portion 26 thereof as shown in FIG. 4. To this end, the opening in the insulation 14 is complementary in con figuration to the current limiting material 26 26'.

With the arrangement illustratedit will be readily appreciated that upon the occurrence of any excessive current, the reduced portion 26' of the current limiting material is first evaporated and then the evaporation proceeds toward the remaining portion 26 thereof with the result that the evaporation is uniformely caused in the direction of the arrow 36.

Within the electrode 20 the piston 24 cooperates with the compressible medium 30 to be effective for causing the evaporation resulting from the preceding evaporation of the reduced material portion 26' to rapidly proceed in the direction of the arrow 36. More specifically, the reduced portion 26' of the current limiting material is first evaporated to establish a very high pressure to compress the compressible medium 30 while moving. the piston 24 in the direction of the arrow 36. It has been found that these operations of the compressible medium 30' and piston 24 are very effective for inducing the evaporation originating from the reduced material portion 26' in the direction of the arrow 36 wherebyto aid and promote in performing an excellent stable current limiting operation. Therefore the arrangement of FIG. 4 ensures an indefinite-number of current limiting operations with a good reproducibleness.

It has been also found that the provision of the reduced material portion 26' disposed adjacent the first electrode 10 is effective for conducting and dissipating heat generatingin that portion with normal magnitude of current flowing therethrough.

FIG. 5 wherein like reference numerals designate the components identical to those illustrated in FIG. 3 shows still another modification of the invention. The arrangement illustrated is identical to that shown in FIG; 3 excepting that the reduce portion 26' of the current limiting material as shown in FIG. 3 is encircled with'a sheath 38 formed of any suitable metallic material high in electric resistivity and good in arc-proof property such as anyof nickel alloys andmolybdenum alloys. The sheath 38 is effective for preventing the insulation 14 from damaging due to an arcing of the reduced-current limiting'portion 26. It has been-found that the-sheath-38 is preferably formed of Monel metal or HASTELLOY (trade mark) having a resistivity in the order of megohms-centimeter.

In summary the invention provides'a current limiting device of self-restoring type including an amount of current limitingmaterial extending through an electric insulatlonformed atlea'stpartly of an electrically insulating'material at least equalin thennal conductivity to the current limiting material. This permits heatgenerated in the current limiting material to be effectively dissipated'and ensures thatthe current'limitingmaterial'is maintained in its original good'electrically conductive state with no evaporationunless a'current flowing'therethrough exceeds a predetermined magnitude.

In order to render the process of evaporating the current limiting material uniform to perform the current limiting operation with a good reproducibleness, the current limitingmaterial may partly reduce in cross sectional area. Also a pressure relief element may be disposed in opposite relationship with respect with the reduced portion of the current limiting material in order to effectively take advantage of the effect of the pressure relief element that the evaporation of the current limiting material is induced. Further the reduced portion of the current limiting material may be encircled with a sheath formed of a material high in resistivity and good in arc-proof property whereby the device performs the stable operation of limiting current while increasing in useful life.

While the invention has been illustrated and described in conjunction with a few preferred embodiments thereof it is to be understood that various changes and modifications may be resorted to without departing from the spirit and scope of the invention. For example, the compressed gas 30 may be replaced by a compression spring.

What we claim is:

l. A current limiting device comprising, a first metallic electrode and a second metallic electrode disposed in spaced opposite relationship, an electric one-piece insulator made of beryllia disposed so as to mechanically connect the first electrode to the second electrode and electrically insulate said first electrode from said second electrode and having a channel extending therethrough and disposed between said first and second electrodes, an amount of current-limiting material filling said channel said current limiting material being in a selected one of a solid and liquid original states at room temperature to maintain the first electrode elec trically connected to the second electrode and responding to a flow of current therethrough in excess of a predetermined magnitude to be evaporated thereby to perform a current limiting operation, said currentlimiting material being self-restoring to its original state when the excess of current flow terminates, at least that portion encircling said current limiting material of said electric insulator comprising a material at least equal in thermal conductivity to said current limiting material, and said second electrode having disposed therein a single pressure relief means in communication with said channel for maintaining a pressure within said channel due to the evaporation of said current-limiting material, below a predetermined value.

2. A current limiting device as claimed in claim 1 in which an end portion of said channel remote from said pressure relief means has a reduced cross sectional area.

3. A current limiting device as claimed in claim 1, wherein an end portion of said channel remote from said pressure relief means has a reduced cross sectional area, and further comprising means disposed circumferentially of said end portion comprising a material having a higher resistivity than that of said currentlimiting material and having good arc-proof properties. I. 

1. A current limiting device comprising, a first metallic electrode and a second metallic electrode disposed in spaced opposite relationship, an electric one-piece insulator made of beryllia disposed so as to mechanically connect the first electrode to the second electrode and electrically insulate said first electrode from said second electrode and having a channel extending therethrough and disposed between said first and second electrodes, an amount of current-limiting material filling said channel said current limiting material being in a selected one of a solid and liquid original states at room temperature to maintain the first electrode electrically connected to the second electrode and responding to a flow of current therethrough in excess of a predetermined magnitude to be evaporated thereby to perform a current limiting operation, said current-limiting material being self-restoring to its original state when the excess of current flow terminates, at least that portion encircling said current limiting material of said electric insulator comprising a material at least equal in thermal conductivity to said current limiting material, and said second electrode having disposed therein a single pressure relief means in communication with said channel for maintaining a pressure within said channel due to the evaporation of said currentlimiting material, below a predetermined value.
 2. A current limiting device as claimed in claim 1 in which an end portion of said channel remote from said pressure relief means has a reduced cross sectional area.
 3. A current limiting deviCe as claimed in claim 1, wherein an end portion of said channel remote from said pressure relief means has a reduced cross sectional area, and further comprising means disposed circumferentially of said end portion comprising a material having a higher resistivity than that of said current-limiting material and having good arc-proof properties. 